Device for measuring the speed of a rail-mounted vehicle

ABSTRACT

A rail mounted vehicle speed measuring device includes a first magnetic field generating member and a first magnetic field sensing member positioned at a first measuring location on the vehicle; a second magnetic field generating member and a second magnetic field sensing member positioned at a second measuring location on the vehicle, spaced a fixed distance from the first location in the direction of vehicle movement; wherein the magnetic fields generated by the first and second magnetic field generating member are influenced by the rail to produce first and second signal patterns sensed by the first and second sensing member and varying with movement of the vehicle along the rail; and device is provided for correlating the first and second sensed signal patterns to determine the time displacement between the two sensed signal patterns and the velocity of the vehicle.

This application is a 371 of PCT/SE 95/00783, filed Jun. 26, 1995.

This application is a 371 of PCT/SE 95/00783, filed Jun. 26, 1995.

TECHNICAL FIELD

The present invention relates to a device for speed measurement in arail-mounted vehicle.

BACKGROUND OF THE INVENTION

It has long been desired to be able to measure the speed of arail-mounted vehicle accurately and with a high reliability over thewhole speed range of the vehicle and under all operating conditions. Itis desirable to be able to obtain an accurate speed value, byintegration, the distance covered by the vehicle and hence the positionof the vehicle along the track, such information being required bysuperordinate traffic control systems. Further, it is desirable toobtain accurate speed value for information to systems for control ofvehicle slip during acceleration or deceleration. In addition, it isimportant to obtain accurate speed measure also at very low speeds.

Also, for reasons of reliability and cost, it is desirable that meansfor speed measurement and position determination be arranged in theirentirety on the vehicle and that they be completely, or to the greatestpossible degree, independent of external means, such as stationarysignalling or measurement systems arranged at the track or at some otherlocation.

The prior art discloses the use of tachometer generators connected tothe wheels of the vehicle. However, slipping of the wheels when drivingor braking entails unavoidable measurement errors with such equipment.Further, the measures of speed and distance obtained from a tachometergenerator are dependent on the current wheel diameter. This changes withtime, both by wear and by the wheels being turned down, which is done atregular intervals. The influence of the diameter change may, to acertain extent, be compensated by recurring calibrations and adjustmentsof the measurement system, but the need thereof entails an essentialdrawback. Under all circumstances, a tachometer generator systemprobably cannot provide a higher accuracy in, for example, distancemeasurement than about 10 to 30 percent.

The article entitled "Hastighetsmatning med korrelationsmetod", Andermo,Mork, Sjolund, Teknisk Tidskrift 1976, No. 3, pages 18-21, has proposed(FIG. 3 with description) that the speed of a rail-mounted vehicle bemeasured optically in a contactless manner with the aid of a correlationmethod. A sensor mounted in a bogie has two light-emitting diodes whichilluminate the rail at two different locations at a known distance fromeach other. The reflected radiation is sensed at both locations with theaid of photodiodes. One of the sensed signals is displaced in time untila maximum correlation is obtained between the two time-variable signals.The time displacement, together with the known distance between themeasuring locations, then determines the speed of the vehicle and, byintegration, also the distance covered. However, in practice, it hasbeen found that optical systems are sensitive to the heavy fouling ofdetectors, etc., which is unavoidable during vehicle operation. Further,particles present between the rail and the sensor, such as, for example,raindrops, snow, and brake dust, result in disturbances of themeasurement, among other things by heavy damping of the optical signals.Therefore, it has proved to be difficult, or impossible, to obtain ahigh reliability and high measurement accuracy during operation invehicle environment using equipment of the above-mentioned type.

In the publication of Jopping, Wennrich: "Radargestutzte Weg- undGeschwindigkeitsmessung auf Schienenfahrzeugen, Signal+Draht, 85 (1993),pages 360-364, a system for speed and road measurement during vehicleoperation with the aid of a Doppler radar is described. Such a systemhas proved to be less sensitive to fouling than an optical system.During vehicle operation in the winter in Nordic or arctic climates,however, the unavoidable presence of snow and ice coating obstructs theradar radiation to such a high extent that the system cannot be usedunder such conditions. Further, in measurement equipment of this kind,it has proved to be difficult to obtain the required accuracy ofmeasurement at low vehicle speeds.

U.S. Pat. No. 4,179,744 describes a device for checking the function ofelectric rail-mounted vehicles. The device has one or more stationarysensors placed along the rail, which are connected to stationarymeasurement amplifiers and signal processing equipment. When the vehiclepasses the sensors, these detect the electromagnetic fields from thetraction equipment of the vehicle. This makes possible control andanalysis of the function of the traction equipment. By arranging twosuch sensors at a known distance from each other along the rail, and byallowing the signal processing equipment to determine the timedisplacement between the signals from the two sensors, the speed of thevehicle may be calculated. The device requires stationary installationsand it may only provide information about the vehicle speed at themoment when the vehicle passes the sensors and cannot provide thecontinuous speed information which is required for, for example,position determination or slip control.

U.S. Pat. No. 4,283,031 describes a device for use in connection withrailway crossings. It disclose the use of stationary sensors arrangedalong the rail for determining, for example, the length of the train,the number of cars, the train speed and direction. Each sensor isarranged near the rail and senses those changes in an electromagneticfield, generated by the sensor, and caused by wheel passages. Bydetermining the time between the passages by a vehicle wheel past twosensors arranged at a known distance from each other, the train speedmay be determined. The device involves the same disadvantages as thedevice described in the preceding paragraph.

U.S. Pat. No. 5,141,183 describes a device in a handling system (e.g. anoverhead travelling crane or an industrial robot arranged on a trolley)comprising a carriage movable along a rail. The carriage has currentcollectors running along stationary contact rails. On the rails,magnetized strips are arranged which have regions with alternatelyopposite magnetization directions. The current collector is providedwith a sensor which senses the field from the strip and counts theregions which are passed. If the regions have known dimensions, thespeed of the car may be determined. This device also requires stationarymembers (the magnetic strips) and may, therefore, only give speedinformation upon the actual passage of the stationary members.

SUMMARY OF THE INVENTION

The invention provides a device of the kind described in theintroductory part of the description, which, within the whole speedrange of the vehicle, exhibits a high reliability and high accuracy ofmeasurement under very severe operating conditions, and is able to workcompletely independently of means or systems arranged outside thevehicle.

Further, an object of the invention is to provide a device which makespossible a reliable detection of non-movement of the vehicle.

The invention also provides a device which makes possible detection ofrail defects, such as cracks and rail failures.

BRIEF DESCRIPTION OF THE DRAWING

The invention will be described in greater detail in the following withreference to the accompanying FIGS. 1-9, wherein

FIG. 1 shows a sensor means according to the invention;

FIG. 1a shows the means of FIG. 1 viewed from the side;

FIG. 1b shows the means of FIG. 1 viewed from above;

FIG. 1c shows the means of FIG. 1 viewed in the direction of movement ofthe vehicle;

FIG. 1d illustrates a sensor means according to an alternativeembodiment, viewed from above;

FIG. 1e illustrates an example of the mounting of the sensor means onthe vehicle;

FIG. 2a shows a block diagram of a measurement device according to theinvention;

FIG. 2b shows the configuration of the electronic system, arranged inthe sensor itself, in the means according to FIG. 2a.

FIG. 2c shows the configuration of the circuits for signal processing ofeach one of the two sensor signals in the means according to FIG. 2a.

FIG. 2d shows how the device according to FIG. 2a may be supplemented inorder to work alternately at two different frequencies.

FIG. 3 shows in the form of a vector diagram the components of theoutput signal of the sensor coil.

FIG. 4 shows an alternative embodiment of the sensor means according tothe invention, with sensor coils in two directions orthogonal to eachother.

FIG. 5 shows a sensor means with three sensor units and with apossibility of choosing between two different measurement distances.

FIG. 6 shows how the switching between the measurement distances may bemade in the device according to FIG. 5.

FIG. 7 shows an example of how the device according to the invention maybe supplemented for detection of non-movement of the vehicle.

FIG. 8 shows an example of how the device according to the invention maybe supplemented with means for detection of defects in the rail.

FIG. 9 shows an alternative embodiment of the magnetization and sensorcoils of the sensor means.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The examples of electric and mechanical dimensioning informationoccurring in the following description are only approximate.

FIG. 1a shows a sensor means according to the invention. It is arrangedon a vehicle bogie above a rail 2, the longitudinal direction of whichcoincides with the direction of movement of the vehicle and lies in theplane of the paper. The sensor means comprises a housing 1 made ofelectrically conducting material, for example aluminium. Theelectrically conducting house walls provide screening between thesensors and against disturbances from external fields.

The housing 1 has three spaces 11, 12 and 13. In each of the spaces 11and 12 a sensor, G1 and G2, respectively, is arranged. Each sensor has acoil frame 110 and 120, on which a magnetization coil 111 and 121,respectively, is arranged. The magnetization coils have substantiallyvertical longitudinal axes. Each coil has a length of about 80 mm, adiameter of about 22 mm, consists of about 150 turns and is fed with analternating voltage with a frequency of about 100 kHz. In the lowerparts of the coil frames 110 and 120, grooves are milled perpendicularlyto the direction of movement, and in these grooves sensor coils 112 and122 are arranged. Each sensor coil has a height of about 7 mm and awidth (perpendicular to the direction of movement of the vehicle) ofabout 25 mm and consists of about 250 turns. The sensor coils arearranged to be rotatable to a certain extent around axes perpendicularto the plane of the paper for adjustment of the coils such that theirsensing directions become perpendicular to the direction of themagnetizing field.

The distance "d" between the lower part of the sensor means and theupper surface of the rail 2 is, for example, about 50-100 mm. It isadapted with respect to the deflection of the bogie and to the reductionof the diameter of the vehicle wheels which arises when the wheels areturned down which is normally done at certain intervals.

The distance L in the direction of movement between the axes of the twosensors is about 100 mm in the example shown. Each sensor system (withat least two sensors) is measured individually to obtain an equivalent"electrical distance", L'_(EL), which is then stored in a non-volatilememory (e.g. an E² memory). This distance, L'_(EL), is then utilized ascalibration value by the signal processing unit.

In the common space 13 between the two sensors, certain electronicequipment is arranged on a printed circuit-board 131. In the exampledescribed, this electronic equipment includes pre-amplifiers for thesignals from the sensor coils and bandpass filters for the sensorsignals.

The housing 1 is provided with connection devices (not shown) forsupplying voltages to the magnetization coils and for the output signalsfrom the printed circuit-board 131 and with the necessary connectionsbetween the sensor coils and the printed circuit-board.

Further, FIG. 1a shows the coordinate system used hereinafter in thedescription. The X axis of the system is vertical and parallel to thelongitudinal axes of the magnetization coils. The Y axis is parallel tothe longitudinal direction of the rail, and hence to the direction ofmovement of the vehicle. The Z axis is horizontal and perpendicular tothe longitudinal direction of the rail.

FIG. 1b shows a section through the sensor means viewed from above. FIG.1c shows a section through the sensor means viewed from the front.

The magnetization coil of a sensor generates a magnetic alternatingfield, the magnetization field, with a substantially vertical maindirection. Since the coil lies relatively close to the rail, the railwill influence the field. Factors which influence the field are themagnitude of the air gap between the sensor and the rail, the geometryof the rail (e.g. dimensional changes, damage, interruptions), and thepermeability and conductivity of the rail.

During movement of the vehicle, variations in these factors willgenerate correlatable variations of the magnetic field configuration.However, the variations in the field are small compared with themagnitude of the magnetization field. Since the sensor coil of eachsensor is separate, however, it may be oriented in space so as to selectand sense that magnetic component which best represents the changes inthe field caused by these variations in the properties of the rail. Byorienting the sensor coils in the manner shown in FIG. 1, that is,orthogonally to the magnetization field, the sensitivity of the coils tothe strong magnetization field is reduced to a very great extent. Thefield which is orthogonal to the magnetization field will, in this way,constitute a greatly increased percentage of the output signal from asensor coil. In this way, it is possible to increase the sensitivity andthe accuracy of the detection of the magnetic-field variations to a verygreat extent.

FIG. 1d shows, viewed from above, an alternative embodiment of thehousing 1 of the sensor means. The housing comprises an extrudedaluminum profile with two circular parts, which form the spaces 11 and12 for the two sensors and which are separated by one part withplane-parallel walls which form the space 13 for the common electronicunit (the printed circuit-board 131).

FIG. 1e shows an example of the mounting on the vehicle of the sensormeans 1 shown in FIGS. 1a-1c. The sensor means is mounted on theunderside of one of the bogies of the vehicle, namely, the bogie 3 withits two wheel sets 31 and 32.

FIG. 2a shows the sensor means according to FIG. 1 with associatedequipment for feeding the field coils and for signal processing of theoutput signals of the sensor coils. A supply unit SU feeds themagnetization coils 111 and 121 with an alternating voltage with afrequency of about 100 kHz with the aid of a sine-wave oscillator OSCand a power amplifier PA. The output voltages u_(i1) and u_(i2) from thesensor coils 112 and 122 are supplied to the electronic circuits 131₁and 131₂ arranged on the printed circuit-board 131. The output signalsu_(d1) and u_(d2) from these circuits are supplied to signal processingcircuits SB1 and SB2, which generate the digital signals S1 and S2. Eachsuch signal constitutes a measure of the instantaneous value of thephase position of the field sensed by the respective sensor coil. Thesignals S1 and S2 are supplied to a calculating unit CE, which by meansof, among other things, correlation of the two signals, calculatesmeasured values of the speed v of the vehicle and the distances covered.

FIG. 2b shows the configuration of the electronic circuit 131₁. Thesensor coil 112 is connected to a load resistance R1 of 50 kohms. Thevoltage u_(il) is supplied to an amplifier and impedance converter F11with an amplification of 5-10 times and an output impedance of 50 ohms.The output signal of the amplifier is filtered in a bandpass filter BP1for filtering away other signals than those which are derived from themagnetization field, which has a frequency of about 100 kHz. In theexample now described, the filter has a passband with upper and lowerlimit frequencies about 150 kHz and 50 kHz, respectively. The outputsignal from the bandpass filter is designated u_(d1).

The electronic circuit 131₂ is built up in the same way as the circuit131₁.

FIG. 2c shows the configuration of the signal-processing circuit SB1shown in FIG. 2a. The signal u_(d1) is supplied to a phase-locked loopPLL1 with a large time constant, one or a few seconds. This circuit hasa phase position which corresponds to the mean value of the phaseposition of the input signal u_(d1). The circuit generates two outputsignals with the same frequency as the input signal, that is, about 100kHz. An output signal u_(r21) has the same phase position as the inputsignal and is supplied to a circuit AGC1 for control of the workingpoint of the sensor means. A second output signal consists of asquare-pulse train u_(rll) which is phase-shifted 90° from the formeroutput signal and is supplied to an input of an exclusive OR circuitXOR1. The signal u_(r11) serves as a phase-position reference whendetermining the phase position of the voltage generated in the sensorcoil.

The circuit AGC1 is a circuit with a controllable gain. The outputsignal u_(dm1) of the circuit has the same phase position and curveshape as the input signal u_(r21) but a variable amplitude. The input ofthe circuit for control of the amplification is supplied with the signalu_(d1). The peak value of this signal is detected, for example, in anenvelope detector, and controls the amplitude of the output signal ofthe circuit in such a way that the amplitude of the output signalalmost, but not quite, corresponds to the amplitude of the measuredsignal u_(d1). The circuit AGC1 has a large time constant, for exampleone or a few seconds, and the output signal u_(dm1) will therefore havethe same frequency as the measured signal u_(d1) and an amplitude and aphase position which nearly correspond to the mean values of theamplitude and the phase position of the measured signal u_(d1).

In a differential amplifier F21, the signal u_(dm1) is subtracted fromthe measured signal u_(d1) and the difference constitutes the outputsignal u'_(d1) of the amplifier. The circuit PLL1--AGC1--F21 nowdescribed will control the working point of the means such that thecomponent in the output signal of the sensor coil which is caused by themagnetization field is eliminated to the desired extent. In this way,the sensitivity and the accuracy in detection of the field variationscaused by the vehicle movement are increased, which variations are smallcompared with the magnetization field.

It has proved to be suitable to not completely eliminate the voltagecomponent caused by the magnetization field, but to allow the outputsignal of the sensor coil to contain, for example, 100 mV of thiscomponent as a phase reference. The XOR gate requires an input signalu'_(dd1) (FIG. 2c), which, on average, should be 90° phase-shiftedrelative to u_(r11) which is obtained from the circuit PLL1. The lattercircuit has found its phase position substantially from the component ofthe magnetization field. Therefore, a sufficiently large component fromthe magnetization field should also be present in the signal u'_(dd1)and thus also in u'_(d1).

The output signal u'_(d1) of the amplifier F21 is supplied to acomparator CMP1 which emits a logic one if the input signal is largerthan zero and a logic zero in the opposite case. The output signalu'_(dd1) of the comparator, which signal is a square pulse train withthe same phase position and frequency as the input signal u'_(d1), issupplied to a second input of the XOR circuit XOR1.

If the two input signals to the XOR circuit are in phase, the outputsignal of the circuit becomes zero. If the input signals are inanti-phase, the output signal becomes 1. On average, the input signalu'_(dd1) will have the same phase position as the measured signalu_(d1), i.e., the phase difference between the two input signals to theXOR circuit will, on average, be 90°. Therefore, the output signal ofthe circuit will have the value 1/2, that is, the working point will, onaverage, lie in the center of the dynamic range of the circuit, whichentails optimum utilization of the dynamic range.

The output signal φ_(p) of the XOR circuit consists of a pulse trainwith a frequency of about 100 kHz and with a mean value which, onaverage, has the value 1/2 and which may vary between theabove-mentioned limits 0 and 1. In a low-pass filter LP1, the 100 kHzcomponent and harmonics of this component, are suppressed, and theoutput signal φ_(a) of the filter is an analog signal which variesconcurrently with the phase position of the output signal of the sensorcoil. The output signal of the filter is amplified and converted intodigital form in an A/D converter AD1 with the output signal S1.

The signal-processing circuit SB2 in FIG. 2a is built up in the same wayas the circuit SB1 described above.

The signals S1 and S2 from the signal-processing circuits SB1 and SB2 inFIG. 2a are supplied to a correlation unit CE. This suitably comprises amicroprocessor programmed to perform speed determination with the aidof, among other things, correlation of the two signals S1 and S2 and tocalculate, by integration/summation of the speed values, the distancecovered by the vehicle.

Each of the signals S1 and S2 is stored continuously as a sequence of apredetermined number of digital values, which thus always reproduce thevariation of the signal during a certain time prior to the moment inquestion.

A continuous calculation of the correlation between the signals S1 andS2 is made when these are displaced by a varying time interval τrelative to each other. The time displacement τ_(m) which provides thehighest value of the correlation integral is used as one subset for thespeed determination. Further, the result of previous measurements (theprevious history), modelling of the dynamic properties of the vehicle(the train), possibly other (less accurate) speed sensors, as, forexample, a tachometer generator, are used as input data. The evaluationprogram, which is a statistical probability calculation with adaptiveweights of the various input data, then provides an MLE (MaximumLikelihood Estimation) of the instantaneous speed of the vehicle. Thespeed of the vehicle is obtained as ##EQU1## where L'_(EL) is theequivalent "electrical distance" between the two sensors (see FIG. 1with associated description)

τ_(MLE) is the value of t which gives the best possible correlationaccording to MLE.

Further, the device may be simply adapted to determine the direction ofmovement of the vehicle by shifting between S1 and S2 during thecorrelation and investigating in which order between the two signalpatterns the correlation is obtained.

The microprocessor is adapted to carry out correlation analysis with apredetermined frequency, for example 10 measurements per second.

FIG. 2d shows how increased reliability in the speed determination maybe obtained by allowing the sensor means to operate alternately at twodifferent frequencies, for example 70 kHz and 100 kHz. A control signalf_(c) from the calculating member CE switches with a suitableperiodicity, for example between each measurement, the oscillatorfrequency between these two values where necessary, filter circuits etc.in the signal-processing units SB1 and SB2 are also switchedsynchronously therewith. Since the depth of penetration of the fieldinto the rail is different for the two frequencies, the sensed signalpatterns will vary in different ways during the movement of the vehicle.However, the speed values calculated at one frequency shall, inprinciple, correspond to the values which are determined at the otherfrequency. If the values do not correspond; it is possible (if thedifference is small) to form the mean value thereof, or (if thedifference is great) to take this as an indication of a fault in thesensor means.

As an alternative to allowing the sensor means to alternately operate atdifferent frequencies, two or more sensor systems and measurementchannels, operating at different frequencies, may be arranged.

If desired, of course, two or more identical sensor means may be used ona vehicle to obtain a higher availability and increased reliability.

FIG. 3 shows, in the form of alternating-voltage vectors, the outputvoltage u_(i) of a sensor coil, which voltage is composed of the twocomponents u_(ix) and u_(iy). That component in the output signal of asensor coil which is directly caused by the magnetization field may, inpractice, never be eliminated by adjusting the orientation of the coil.However, by the circuit for control of the working point of the means,described above with reference to FIG. 2c, this component may be furtherreduced to the desired degree. However, it has proved to be suitable notto eliminate the component completely, and therefore, in the outputsignal of the sensor coil, there is a component u_(ix) which is causedby the magnetization field. The field variations in the direction ofsensing of the sensor coil (orthogonally to the magnetization field),which are caused by the movement of the vehicle, will substantiallyprovide a component u_(iy) of the output signal of the sensor, whichcomponent has a 90° phase shift relative to the component u_(ix). Thevariations of the component u_(iy) cause variations of the phaseposition φ of the output signal of the coil relative to the phaseposition of the component u_(ix) generated by the magnetization field.As mentioned, the orientation of the sensor coils orthogonally to themagnetization field entails a great reduction of the influence of themagnetization field on the output signal of a coil. The variations inthe phase position of the sensor signal which are caused by thevariations in the voltage component u_(iy) therefore become greatlyincreased, which entails a good sensitivity and accuracy in thedetection.

FIG. 4 shows, viewed from above, a sensor means of the same type as thatshown in FIG. 1. A sensor, G1 and G2, respectively, is arranged in eachof the spaces 11 and 12 and each sensor has, in the same way as in FIG.1, a magnetization coil (not shown) which generates an alternating fieldwith a vertical main direction. Also, in the same way as in FIG. 1, eachsensor has a sensor coil 112y and 122y, respectively, with their sensingdirections in parallel with the Y-axis. In the means shown in FIG. 4,each sensor has an additional sensor coil, 112z and 122z, respectively,with its sensing direction in parallel with the z-axis.

From the sensor coils 112y and 122y, two sensor signals, here designatedu_(dly) and u_(d2y), are obtained, in the same way as described withreference to FIGS. 1 and 2, via electronic circuits 131 arranged in thesensor, which sensor signals are signal-processed and correlated witheach other to form a measure of the speed v of the vehicle in the mannerdescribed with reference to FIG. 2. The signals u_(d1z) and u_(d2z) areprocessed in the same way, either by separate signal-processing circuitsand calculating means, or by using the same circuits alternately fordetermining the vehicle speed from the signals from one of the pairs ofcoils and alternately from the signals from the other pair of coils.Possibly, one of the pairs of coils with its signal-processing circuitsand calculating means may be used in the normal case and the other pairof coils with its signal-processing circuits and calculating means serveas a pair of stand-by coils to be activated in the event of a fault inthe normally used system.

It has been found that sensor coils with their sensing direction in they-direction are insensitive to fields which are caused by traction andsignal currents flowing in the rail, and that this coil orientation maybe preferable. This is not the case with coils which have their sensingdirection in the z-direction, but the disturbing influence of theabove-mentioned currents may to a great extent be reduced with the aidof some known disturbance elimination method, for example according toSwedish patent having publication number 441 720.

FIG. 5 schematically shows a sensor unit according to an alternativeembodiment of the invention. It has three sensors, each designed as, forexample, the sensors in FIG. 1. It has a sensor G1 and a sensor G2 inthe same way as the sensor means of FIG. 1. The distance between thecenter lines of the sensors constitutes the measuring distance L. Thesensor signals from the sensors G1 and G2 are correlated with each otherin the manner described with reference to FIG. 2, and the speed of thevehicle is calculated by means of the measuring distance L.

Between the sensor G1 and the common electronic space 13, a third sensorG2' of the same kind is arranged adjacent the sensor G1 and formstogether therewith a shorter measuring distance L' with a length of, forexample, 40 mm. At low vehicle speed, this shorter measuring distanceprovides considerably faster speed determination than the longermeasuring distance L. As shown in FIG. 6, the choice of measuringdistance may preferably be made automatically in dependence on vehiclespeed. The signals u_(d2) and u'_(d2) from the sensors G2 and G2',respectively, are supplied to the signal-processing unit SB2 viaelectronic switching members SW1 and SW2. A level-sensing circuit NV1 issupplied with the calculated speed value v. If the speed is greater thana certain predetermined value v₀, the longer measuring distance is used,and the signal u_(d2) is switched via the switching member SW1 into thesignal-processing unit SB2. If the speed does not exceed the value v₀,the shorter measuring distance is activated by instead switching thesignal u'_(d2) via the switching member SW2 into the unit SB2.

Although the system described above with a suitable dimensioning mayprovide a good measurement result down to a very low speed, themeasurement system unavoidably ceases to function when the speedapproaches zero. In many applications, therefore, it is desirable tocomplete the system with an indication as to whether the speed of thevehicle is zero.

When the vehicle is stationary, S1 and S2 will be uncorrelated timesequences for all time displacements τ between the sequences. One methodis to test for total independence when the estimated value of the speedis below a predetermined limit. When this hypothesis is verified at agiven significance level, uncorrelated sequences are indicated. Inaddition, S1 and S2 will be approximately static sequences when thespeed is zero. A low variance (RMS value) is, therefore, also anindication of the speed being zero.

FIG. 7 shows how these tests may be combined. The calculating unit CEais supplied with the signals S1 and S2 and delivers, if theabove-mentioned test of the absence of correlation is fulfilled, asignal NC which indicates that the two signals are uncorrelated. Thecircuits CEb and CEc deliver signals LV1 and LV2 if the variance of thesignals S1 and S2, respectively, lies below predetermined levels. Thesignals NC, LV1 and LV2 are supplied to an AND circuit AC which deliversan indicating signal V=0 if all three tests are fulfilled.

As an alternative to the tests described in the preceding paragraph,other known statistical standard tests for stationary state may be used.

The circuits shown in FIG. 7 suitably consist of parts of the programfor a microprocessor which constitutes a control and calculating unitfor the speed sensor.

FIG. 8 shows how the device described above may be supplemented withmeans for detection of defects in the rail, such as cracks or rupture.The sensor signal S1 is supplied to a circuit CD which calculates one ormore predetermined characteristics C_(S1) of the signal, for examplemaximum amplitude or rate of change. That value, or those values C_(M)of the corresponding characteristics, which occur at the defect ordefects which are to be detected, are stored in advance in a memory M. Acomparison circuit COMP2, a pattern recognition circuit, continuouslycompares the characteristic quantities C_(S1) and C_(M) and delivers adetection or alarm signal SL at a predetermined degree ofcorrespondence.

Alternatively, the defect detection may be made by comparing that signalpattern, which the values of S1 for a certain period of time constitute,with the corresponding signal pattern stored in advance in the memory.The comparison between the signal patterns may possibly be made bytime-shifting one of the patterns, in the same way as with the speedmeasurement described above, in relation to the other pattern untilmaximum correlation is obtained, whereby a fault is considered to havebeen detected if at least a predetermined degree of correlation isobtained.

FIG. 9 schematically shows an alternative and advantageous embodiment ofthe magnetization and sensor coil of a sensor (e.g. G1 or G2 in FIG. 1).The magnetization coil 111 is designed as a flat sheet-wound coil with avertical axis. The coil has a considerably smaller length (extent in thevertical direction in the figure) than its diameter. By making the coilshort, all its winding turns will be as close to the rail as possible,which provides a high magnetizing field intensity at the rail surface.The sheet winding is suitably performed in the manner shown in thefigure, with a large number of turns of a thin sheet, which provides alarge effective area and hence a lower effective resistance and a highercurrent-handling capacity than a corresponding wire winding, since atthe frequencies used, the depth of penetration will be small because ofthe skin effect. Alternatively, however, the magnetization coil may, ofcourse, be designed as a wire-wound coil.

The sensor coil 112 is arranged adjacent to the magnetization coil, andat the same height d above the rail 2 as this. As in the sensorsdescribed above, the sensor coil has a horizontal longitudinal axis andsensing direction. With the location shown, as well as with the sensorsdescribed above, the sensing direction of the coil will be perpendicularto the direction of the magnetizing field at the sensor coil. Since thesensor coil is arranged adjacent to the magnetization coil, the lattermay be arranged nearer the rail, which provides a higher field intensityat the rail surface. It has proved to be particularly advantageous toarrange the sensor coil 112 displaced in the longitudinal direction ofthe rail by a distance from the magnetization coil which isapproximately half (d/2) of the distance d between the magnetizationcoil 111 and the rail. At this distance, the magnetizing field intensityat the rail is highest, and a maximum induction is obtained in thesensor coil. The embodiment of the sensor shown in FIG. 9 has proved toprovide good detection properties.

In the sensor means described above, the signals which are correlatedare formed by sensing the phase position of the sensor signal.Alternatively, the amplitude component of the sensor signal may also beused.

Further, in the sensor means described above, each of the signals whichare correlated is obtained from one single sensor coil. Alternatively,several sensor coils may be connected together to form such a signal, inwhich case the coils are oriented with different sensing directions,chosen in a suitable manner, for optimization of the sensitivity of thetotal output signal to the desired field variations and/or insensitivityto direct influence by the magnetization field.

For monitoring the function of the device, a separate monitoring windingmay be adapted to sense the amplitude and/or phase of the magnetizationfield. In that case, a monitoring unit is adapted to trigger an alarm inthe event of loss of the magnetization field, or if the characteristicsof the field deviate from the desired ones.

We claim:
 1. A rail mounted vehicle speed measuring device comprising:afirst magnetic field generating means and a first magnetic field sensingmeans positioned at a first measuring location on the vehicle; a secondmagnetic field generating means and a second magnetic field sensingmeans positioned at a second measuring location on the vehicle, spaced afixed distance from the first location in the direction of vehiclemovement; wherein the magnetic fields generated by the first and secondmagnetic field generating means are influenced by the rail to producefirst and second signal patterns sensed by the first and second sensingmeans and varying with movement of the vehicle along the rail; and meansfor correlating the first and second sensed signal patterns to determinethe time displacement between the two sensed signal patterns and thevelocity of the vehicle.
 2. A device for speed measurement in arail-mounted vehicle comprising:at least two sensors (G1, G2) which arearranged on the vehicle at a known distance (L) from each other in thelongitudinal direction (Y) of the vehicle, each sensorincluding:field-generating means for generating a magnetizationalternating field, surrounding the rail, and comprising a magnetizationcoil supplied with alternating current, and at least one sensor coilseparated from the magnetization coil, for sensing field variationswhich are caused by the movement of the vehicle and being arranged withits sensing direction (Y) substantially perpendicular to the directionof the magnetization field, and members arranged on the vehicle andadapted to be supplied with the output signals (u_(i1), u_(i2)) of thesensor coils, to form for each sensor a signal pattern (S1, S2) whichcorresponds to a time variation of the sensed field caused by themovement of the vehicle along the rail, to thereby determine, bycorrelation of the two signal patterns, the time displacement (t_(m))between them, and to determine, on the basis of said time displacementand on the basis of the known distance between the measuring locations,the speed (v) of the vehicle.
 3. A device according to claim 2, whereinthe field-generating means generates alternating fields with a frequencyexceeding 10 kHz.
 4. A device according to claim 1, wherein thefield-generating means generates alternating fields with one of at leasttwo optional different frequencies (f₁, f₂).
 5. A device according toclaim 2, wherein the field-generating means generates alternating fieldswith one of at least two optional different frequencies (f₁, f₂).
 6. Adevice according to claim 4, wherein the field-generating alternatelyoperates at two different frequencies (f₁, f₂).
 7. A device according toclaim 2, wherein the magnetization coils are arranged with theirlongitudinal axes (X) substantially perpendicular to the longitudinaldirection (Y) of the rail.
 8. A device according to claim 7, wherein themagnetization coils are arranged with their longitudinal axes (X)substantially vertical.
 9. A device according to claim 2, wherein ineach sensor the sensor coil is arranged between the magnetization coiland the rail.
 10. A device according to claim 2, wherein the sensorcoils are arranged with their sensing directions (Y) substantiallyhorizontal.
 11. A device according to claim 10, wherein the sensor coilsare arranged with their sensing direction (Y) substantially parallel tothe longitudinal direction of the rail.
 12. A device according to claim10, wherein the sensor coils are arranged with their sensing directions(Z) substantially perpendicular to the longitudinal direction of therail.
 13. A device according to claim 2, wherein the magnetization coilsand the sensor coils are ironless air coils.
 14. A device according toclaim 2, wherein the output signal (u_(i1)) of each sensor coil isadapted to be supplied to means for sensing variations in the phaseposition (φ) of the signal.
 15. A device according to claim 14, whereinmeans for sensing variations in the phase position of the signalcomprises a phase-locked loop for generating a phase reference signal(U_(r11)).
 16. A device according to claim 14, wherein the output signal(u_(i1)) of the sensor coil is adapted to be supplied to means forelectronic control of the working point of the means for sensingvariations in the phase position of the signal.
 17. A device accordingto claim 16, wherein the means for electronic control of the workingpoint comprise means for generating a signal (u_(dm1)) corresponding tothe mean value of the sensor signal and means for subtraction of saidsignal from the instantaneous value (u_(d1)) of the sensor signal.
 18. Adevice according to claim 2, further comprising a first sensor and asecond and a third sensors arranged at different distances, from thefirst sensor and, selector means adapted to select, for correlation withthe output signal (u_(d1)) from the first sensor, the output signal(u_(d2), u'_(d2)) from one of the second and the third (G2') sensors.19. A device according to claim 18, further comprising means forautomatic selection of the output signal from the second or the thirdsensor in dependence on the speed (v) of the vehicle.
 20. A deviceaccording to claim 2, wherein the sensed signal patterns are adapted tobe supplied to means for detection of non-movement of the vehicle.
 21. Adevice according to claim 20, wherein means for detection ofnon-movement of the vehicle comprise means for detection of the absenceof variation of a signal pattern.
 22. A device according to claim 20,further comprising means for detection of the absence of correlation ofthe signal patterns from two different sensors.
 23. A device accordingto claim 2, further comprising means for storage of characteristics of asignal pattern occurring at a rail defect and means for detection ofrail defects by continuous comparison between said storedcharacteristics and the corresponding characteristics of a signalpattern (S1) sensed during the movement of the vehicle.
 24. A deviceaccording to claim 2, wherein the sensors are mounted on a vehiclebogie.
 25. A device according to claim 2, wherein the extent of themagnetization coil in its longitudinal direction is considerably smallerthan the diameter of the coil.
 26. A device according to claim 25,wherein the magnetization coil is a sheet-wound coil.
 27. A deviceaccording to claim 2, characterized in that the sensor coil is arrangedat substantially the same vertical distance from the rail as themagnetization coil.
 28. A device according to claim 27, wherein thesensor coil is displaced in the longitudinal direction of the rail inrelation to the magnetization coil by a distance which constitutesapproximately half of the distance between the magnetization coil andthe rail.